1. Field of the Invention
The present invention generally relates to a method for timing control, and more particularly, to a method for timing control of channel estimation.
2. Description of Related Art
In a wireless communication environment, a Doppler shift caused by movement of a receiving end or a transmitting end or a multipath phenomenon caused by diffraction and refraction of electromagnetic wave traveling through obstacles between the receiving end and the transmitting end makes the received signal at the receiving end interfering by huge interference. Taking the current code division multiple access system (CDMA system) as an example, although the CDMA system transmits signals of multiple users simultaneously by using different spreading codes respectively corresponding to multiple users and prevents the signals of different users from interfering mutually based on the orthogonality between spreading codes; however, when a transmitted signal in the transmission channel suffers multipath interference, the orthogonality between spreading codes will be destroyed and introduces multiple-user interference (MUI).
In order to improve transmission quality, a CDMA system employs a channel estimation approach at the receiving end thereof to estimate response of the transmission channel and uses an equalizer to eliminate the effect of the transmission channel on the received signal. FIG. 1 is a systematic block diagram of a receiving end with an equalizer of a conventional CDMA system. Referring to FIG. 1, a channel estimation unit 110 estimates a channel impulse response through a received signal r(t), which means the delay time τ of each delay path and the corresponding channel gain ĥ(τ) of each delay time in the channel are estimated, and then the channel estimation unit 110 outputs the estimated channel gain ĥ(τ) to the equalizer 130. The equalizer 130 is used to eliminate the interference contained in the received signal r(t). After that, a correlator 150 despread the received signal after being proceeded by the equalizer 130 with the spreading code c[n] of a user terminal, and furthermore a decision unit 170 demodulate the despreaded received signal to obtain a digital signal {circumflex over (b)}.
However, for some channels with larger delay spread, the channel estimation unit 110 does not necessarily consider all the delay paths in the channel due to hardware limitation as shown by FIG. 2. FIG. 2 is a channel power-delay profile. Referring to FIG. 2, the abscissa herein represents delay time τ and the ordinate represents channel power, i.e. |ĥ(τ)|2. FIG. 2 contains six delay paths Path_1-Path_6, and six pulses herein represent the delay time and the powers of the delay paths Path_1-Path_6. In addition, a channel estimation window (CE window) is shown in FIG. 2 as well, wherein the length of the CE window represents the length of delay time that is able to cover whenever channel estimation processes. It can be seen from FIG. 2, the length of the CE window is less than the delay spread of the multipath. As a result, in a channel power-delay profile, no matter how the CE window is placed, the CE window is unable to cover all the delay paths Path_1-Path_6.
Based on the current technique level, there are two placing ways of CE window. The first way herein is based on the total channel power within a CE window, that is, to place a CE window at a position able to contain the maximum channel power, wherein the total power is the channel power summation of all the delay paths within the CE window. The second way is to directly define the starting position of signal frame, i.e. the first delay path arriving the receiving end, as the starting position of the CE window as shown by the placing position of the CE window in FIG. 2. Note that the delay paths Path_5 and Path_6 in FIG. 2 are not contained within the CE window. In other words, restricted by the hardware, a CE window is unable to cover all delay paths in a channel, which makes the channel estimation approach fail to estimate the real channel impulse response and further degrades the performance of the equalizer. Therefore, the position placing of a CE window is an important index to indicate the quality of channel estimation and further an important factor affecting the performance of the receiving end.
In the present patents however (for example, U.S. Pat. No. 7,058,399 and US patent publication No. 20070009065), only the arranging method of a multipath searcher (MPS) window in a CDMA system is disclosed, but nothing regards how to properly arrange a CE window.